A blue-violet semiconductor laser diode, made of Group III-V nitride semiconductors AlxGayIn1-x-yN (where 0≦x≦1 and 0≦y≦1) such as gallium nitride (GaN), is a key device that would enable optical disc drives to perform a write operation with extremely high densities, and is going to be introduced into actual products in increasing numbers. To develop new industrial fields of applications, it is indispensable to further increase the output power of such a blue-violet semiconductor laser diode because that would contribute to not only getting writing on optical discs done at even higher speeds but also applying it to laser displays.
In the prior art, a sapphire wafer has been used to make nitride semiconductor devices as disclosed in Patent Documents Nos. 1 and 2. When a sapphire wafer is used, an AlGaN based buffer layer, including Al atoms, is sometimes formed on the sapphire wafer to reduce the strain resulting from a lattice misfit with nitride semiconductor crystals or a difference in coefficient of thermal expansion.
On the other hand, if a light-emitting element structure made of nitride semiconductors is formed on a GaN wafer, which has been recently used more and more often in place of the sapphire wafer, a GaN based homo-epitaxially grown layer is usually formed at an initial stage of the crystal growing process.
If a blue-violet light-emitting element is fabricated using nitride semiconductors, however, InN included in InGaN based crystals that form its active layer easily decomposes thermally. That is why a GaN or AlGaN based undercoat layer, including no In atoms, is ordinarily grown at approximately 1,000° C. and then the growth is halted to decrease the temperature to approximately 800° C. And it is known that the surface of that GaN or AlGaN based undercoat layer roughens when the growth is halted abruptly that way.
To overcome such a problem, it was proposed in Patent Document No. 3, for example, that an AlN layer having a higher decomposition temperature than the GaN layer (e.g., an AlGaN undercoat layer including at least 10% of Al atoms, in particular) be formed in advance as a planarity maintaining layer, thereby reducing that surface roughening due to the sudden halt to the growth and growing InGaN based crystals of quality.
Patent Document No. 4 discloses that in growing an n-type cladding layer or an active layer of nitride semiconductors on a wafer, the closer to the active layer a given layer is, the lower the concentration of an n-type dopant to be introduced into that layer should be. By adopting such a configuration, the carriers that have been injected through an electrode can be uniformly diffused more easily, and a light-emitting diode (LED) that produces emission uniformly in a broad area can be fabricated.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2002-252177        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 10-150245        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2000-156544        Patent Document No. 4: Japanese Patent No. 2785254        